Preparing for Plaster Walls: Drywall

With the interest in plaster walls being so high, people are wanting to install more and more of it in their homes and offices. Plaster can be used over almost any substrate, though the preparation will vary depending on the type of surface we are covering, the type of plaster we are using, and the type of performance we are wanting. The common factors of any wall we want to finish with plaster is that it must be stable, firm and dry. Lets look at some of the more common wall types and see what kind of preparation needs to be done.

Drywall or Sheetrock: Lets start with the most common wall surface in the United States and Canada, drywall. Drywall provides a great surface for most if not all plasters because it is so stable. It does not move much with changes in humidity and temperature. Assuming (and sometimes this is where the problem lies) the framing is modern and up to code drywall should be a perfect fit for plaster. One big mistake in new drywall preparation is not sealing the wall using a PVA primer, or not using a manufacturers primer and trying to go right over drywall mud. Depending on the drywall compound that was used the moisture content of the plaster may reactivate the mud and we will get the plaster delaminating. If a setting compound was used (which I recommend) we still have to be concerned with uneven suction with varying thicknesses of the mud, and we can see ghosting in the plaster where the taped seams and nail holes are visible because of the way those areas with deeper mud draws more moisture out of the curing plaster and affects the look of the plaster. Both of these issues can be avoided by simply sealing the drywall with the appropriate primer. After that sealing primer is put up then you may proceed with the plaster manufactureres primer and preparation steps. Some manufactures may combine the PVA with an aggregate (sand) and this can result in a single step, just make sure to follow the advice of the manufacturer.

Another big mistake for new drywall is the descrepancy between the drywall mud preparation and the level of finish desired, i.e. we want a smooth finish but the drywall has only been finished to a level three or even two. The smoother a plaster finish the smaller the aggregate in it is and there fore the more topographical varioation in the drywall mud coat will show thru. I recommend smoothing a wall a smooth as you can get it, finish plasters are just that, finish plasters. They aren't designed to bridge gaps, fill holes, or hide unisightly trowel marks. Of course if you are wanting a more rustic finish, the drywall prep does not need to be a level 5. Be smart and take the prep as far as you need to achieve the top coat you are looking for. Talk to your plasterer, they can help navigate prep if they are not doing it.

Regarding previously painted drywall, clean it, use the appropriate primer for the sheen of the existing paint and then move on to the primer step in the plaster manufacturers recommendation. Don't forget to assess the texture of the wall, if you have a knockdown texture and want to smooth out the surface, remember, finish plaster or veneer plasters aren't designed to fill in that topography. Heavily textured walls will need to be skim coated either using a drywall compound, or larger aggregate plaster before your finish plaster.